There is widespread interest in developing photosensitive organic semiconductor compositions for use in various optoelectronic devices, including photovoltaics (solar cells) and visible light photodetectors. Such optoelectronic devices typically incorporate a p/n heterojunction formed at the interface between p-type and n-type semiconducting layers. For organic materials the designation “n-type” indicates the material to exhibit electron accepting character, and as such is considered capable of transporting electrons. The measure of electron accepting character is usually provided either by the electron affinity or electrochemical reduction potential of the organic material. A “p-type” organic material, on the other hand, is considered to have electron donating character and as such is considered to be “hole transporting”. The measure of electron donating character is usually provided by the ionization potential or electrochemical oxidation potential of the organic material. The simplest organic p/n heterojunction comprises a bilayer formed by the superposition of electron donor and electron acceptor semiconducting organic layers. Multiple organic donor/acceptor layers can be stacked to give a more complex design. Bulk heterojunctions have also been described, wherein the n-type and p-type organic semiconductor materials are blended to yield multiple donor/acceptor interfaces.
A principal feature of the p/n heterojunction is the built-in potential at the interface between the p-type (donor) material and the n-type (acceptor) material. To a first approximation this built-in potential is the origin of the rectifying nature, which arises from the differences in the ionization potentials and electron affinities of the two materials which make up the heterojunction. When electrons and holes are photogenerated in the vicinity of the junction, the field due to the built-in potential serves to separate the charge. The charge separation at the interface is, therefore, the origin of the photovoltaic effect. Such p/n heterojunction diodes can serve as photodiodes and as the fundamental element in a photovoltaic cell, commonly known as a solar cell.
A variety of photoactive organic molecules, crystals, pigments, conjugated and non-conjugated polymers, oligomers, and composites have been developed for use as semiconductor donor and acceptor materials for photovoltaic and photodetector applications. There are several guidelines in designing or selecting photoactive organic materials for such use. First, it is desired that the material have a high optical absorption coefficient α (alpha) for incident electromagnetic radiation. For photovoltaic (solar cell) and visible light photodetector applications, in particular, it is important for the organic material to exhibit a high optical absorption coefficient for visible and near infrared radiation so that very thin layers of the photosensitive organic material can be used to absorb nearly all of the incident radiation. Second, it is desired that the exciton (i.e., the excited state electron-hole bound pair) created by the process of light absorption by the photosensitive organic material have a long diffusion length L such that the exciton can migrate through the respective layer and reach the donor/acceptor (p-n) heterojunction before geminate recombination of quenching occurs. Third, it is desired that upon reaching the interface the exciton disassociates into electrons and holes due to the difference in ionization potential of the donor and the electron affinity of the acceptor. The latter process can be viewed as an exothermic chemical reaction, i.e., a reaction in which some energy is released as vibrational energy. This reaction occurs because the energy separation of the dissociated exciton, i.e., the energy difference between the free electron in the acceptor material and the free hole in the donor is smaller than the energy of the exciton prior to dissociation. Fourth, it is desired that the electron and holes created upon excition dissociation at the donor/acceptor interface have a high mobility in their respective layers so that they may be separately collected at opposing contacting electrodes and contribute to the photocurrent. Lastly, it is desired that the photosensitive organic material be easily processed to form the appropriately thin layers or blends of donor and acceptor component.
Unfortunately, prior art formulations of organic semiconducting materials have typically suffered from one or more disadvantages with regards to their optical or electronic materials properties and/or to their convenience for processing that have significantly restricted their use.
For example, a wide variety of “small” molecule materials and pigments have been used to fabricate p/n heterojunctions for photovoltaic and photodetector applications. References to the use of small molecule and pigment materials include C. W. Tang Appl. Phys. Lett. (1986) 48, 183, Peumans, P.; Yakimov, A.; Forrest, S. J. App. Phys. (2003), 93(7), 3693, Petritsch, K.; Dittmer, J. J.; Marseglia, E. A.; Friend, R. H.; Lux, A.; Rozenberg, G. G.; Moratti, S. C.; Holmes, A. B. Solar Energy Materials and Solar Cells (2000), 61(1), 63-72, all incorporated herein by reference. Typical of such materials are “electron-donor” copper phthalocyanine (CuPc) and “electron-acceptor” 3,4,5,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI). Thin films of these materials can be fabricated with thermal evaporation, chemical vapor deposition (CVD) and so on. Simple donor/acceptor bilayers, as well as multiple alternating layers of donor and acceptor, can be assembled. However, vacuum sublimation is a batch process, which makes production scale runs quite costly, and thin, sublimed films are fragile and susceptible to damage. Alternatively, soluble derivatives of these materials can be prepared and dissolved in an appropriate solvent such that thin films can be fabricated by casting directly from solution or using similar fluid phase processing. Unfortunately, the films of molecular organic donor or acceptor materials prepared by vacuum or solution processing methods tend to be amorphous (non-crystalline) and, as such, generally exhibit small exciton diffusion lengths and low carrier mobility. Consequently, photovoltaic devices and photodetectors constructed from these amorphous materials exhibit relatively low photon-electric conversion efficiency (low quantum yield), being on the order of 2% or less.
Also, a variety of polymeric materials have been used to fabricate p/n heterojunctions for photovoltaic, photodetector, and other optoelectronic applications. Photosensitive junctions can be produced using two semiconducting organic layers in a donor/acceptor heterojunction (i.e., bilayer) structure or alternation layer structures. Conducting polymers which combine the electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantages of polymers have been described by A. J. Heeger, S. Kivelson, J. R. Schrieffer, W. P. Su, Review of Modern Physics 60, 781 (1988), incorporated herein by reference. The ability to control the energy gap and electronegativity through molecular design has enabled the synthesis of conducting polymers with a range of ionization potentials and electron affinities; See T. A. Skotheim, Ed., Handbook of Conducting Polymers Vol.-T, X-T (Marcel Dekker, New York 1986), J. L. Bredas and R. R. Chance, Eds., Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics and Molecular Electronics (Kluwer Academic Press, Netherlands 1990), both incorporated herein by reference. The active layer can comprise one or more semiconducting, conjugated polymers, alone or in combination with non-conjugated materials, one or more organic molecules, or oligomers. The active layer can be a blend of two or more conjugated polymers with similar or different electron affinities and different electronic energy gaps. The active layer can be a blend of two or more organic molecules with similar or different electron affinities and different electronic energy gaps. The active layer can be a blend of conjugated polymers and organic molecules with similar or different electron affinities and different energy gaps. The latter offers specific advantages in that the different electron affinities of the components can lead to photoinduced charge transfer and charge separation; a phenomenon which enhances the photosensitivity [N. S. Sariciftci and A. J. Heeger, U.S. Pat. No. 5,333,183 (Jul. 19, 1994); N. S. Sariciftci and A. J. Heeger, U.S. Pat. No. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz, N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger and F. WudI, Phys. Rev. B 47, 13835 (1993); N. S. Sariciftci and A. J. Heeger, Intern. J. Mod. Phys. B 8,237 (1994), all incorporated herein by reference]. The active layer can also be a series of heterojunctions utilizing layers of organic materials or blends as indicated above.
Typical p-type (donor) semiconducting polymers are poly-3-hexylthiophene (PT) and poly(2-methoxy, 5-(2′-ethyl-hexyloxy) paraphenylenevinylene (MEH-PPV). Other examples of typical semiconducting conjugated polymers include, polyacetylene, (“PA”), and its derivatives; polyisothianaphlene and its derivatives; polythiophene, (“PT”), and its derivatives; polypyrrole, (“PPr”), and its derivatives; poly(2,5-thienylenevinylene), (“PTV”), and its derivatives; poly(pphenylene), (“PPP”), and its derivatives; polyflourene, (“PF”), and its derivatives; poly(phenylene vinylene), (“PPV”), and its derivatives; polycarbazole and its derivatives; poly(1,6-heptadiyne); polyisothianaphene and its derivatives; polyquinolene and semiconducting polyanilines (i.e. leucoemeraldine and/or the emeraldine base form). Representative polyaniline materials are described in U.S. Pat. No. 5,196,144, incorporated herein by reference. Bilayer p/n junction or blended (bulk) p/n heterjunction structures have been produced by layering or blending the donor semiconducting polymer with an acceptor material of n-type (acceptor) poly(cyanophenylenevinylene) (“CN—PPV”). Alternatively, fullerene molecules such as C 60 and its functional derivatives (such as PCBM), or organic molecules have been used an acceptor material of n-type (acceptor). The use of conjugated polymers as photosensitive materials has been described for example in the following reports: S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synth. Metals 54, 427 (1993); H. Antoniadis, B. R. Hsieh, M. A. Abkowitz, S. A. Jenekhe, M. Stolka, Synth. Metals 64, 265 (1994); G. Yu, C. Zhang, A. J. Heeger, Appl. Phys. Lett. 64, 1540 (1994); Friend, R. H. Pure Appi. Chem. (2001) 73, 425-430; R. N. Marks, J. J. M. Halls, D. D. D. C. Bradley, R. H. Friend, A. B. Holmes, J. Phys.: Condens. Matter 6, 1379 (1994); A. J. Heeger and G. Yu, U.S. Pat. No. 5,504,323 (April, 1996); R. H. Friend, A. B. Homes, D. D. C. Bradley, R. N. Marks, U.S. Pat. No. 5,523,555 (June, 1996). [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994); G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl. Phys. 78, 4510 (1995)], all incorporated herein by reference. Unfortunately, optoelectronic devices such as solar cells and photodetectors constructed using semiconducting polymers materials exhibit relatively low photon-electric conversion efficiency (low quantum yield).
On the other hand, it is known that the electronic and optical properties of organic semiconductors improve with increasing structural order. For example the exciton diffusion length L in single crystal naphthalene and anthracene is reported to be about 200 nm compared to less than 100 nm for many noncrystalline organic molecular films. Powell, R.; Soos, J.; J. Lumin. (1975) 11, 1-45, Meth, J.; Marshall, C.; Fayer, M.; Solid State Commun., (1990) 74, 281-284., Peumans, P.; Yakimov, A.; Forrest. S. J. App. Phys. (2003), 93(7), 3693, all incorporated herein by reference. The performance of amorphous organic semiconductor films is known to be enhanced by solvent and thermal annealing processes which tend to crystallize organic semiconductor materials. An enhancement in quantum efficiency has been demonstrated for example by solvent annealing to produce a crystal network in an organic solar cell containing a film of n-type (acceptor) perylene bis(phenethylimide) (Dittmer, J. J.; Lazzaroni, R.; Leclere, Ph.; Moretti, P.; Granstrom, M.; Petritsch, K.; Marseglia, E. A.; Friend, R. H.; Bredas, J. L.; Rost, H.; Holmes, A. B.; Solar Energy Materials and Solar Cells (2000), 61(1), 53-61, incorporated herein by reference.) Solvent treatment also improved the crystallinity and photovoltaic properties of a p-type magnesium phthalocyanine film (Hor, A. M.; Loufty, R. O. Thin Solid Films 1983, 106, 291-301). The exciton diffusion length L in films of n-type (acceptor) perylene bis(phenethylimide) have also been shown to increase upon crystallization by solvent annealing. (Gregg, B. J. Phys. Chem. (1996) 100, 852-859, incorporated herein by reference.) Carrier mobility in crystal networks is also much higher than in amorphous compositions. For vapor deposited layers Kotani has shown that carrier mobilities are strongly dependent on the substrate temperature during the vapor-deposition process, wherein high substrate temperature afforded increase film crystallinity and higher carrier mobilities increased nearly ten-fold (Kotani, T. et. al., In “Proceedings of the Eleventh International Congress on Advances in Non-Impact Printing Technologies”, M. Hopper, ed., IS&T, Springfield, Va., ;42, 1995). Unfortunately, control over the morphology and extent of crystallization by such annealing processes is poor. Additionally, such solvent annealing processes can lead to formation of unwanted pinholes in the crystallized layer.
Organic semiconductor crystals have been successfully formulated for use as photoconductors for electrophotography as described in “Photoreceptors: Organic Photoconductors”, P. Borsenberger, D. Weiss, in Handbook of Imaging Materials, 2nd edition, Diamond and D. Weis, Ed, Marcel Dekker, New York 2002, pp 368-424. Compositions comprising finely-divided, crystalline n-type (acceptor) semiconductor pigment material perylene bis(phenethylimide) dispersed in an inert polymeric binder have been described, for example, by Gruenbaum et al. U.S. Pat. No. 4,968,571, and U.S. Pat. No. 5,019,473 incorporated herein by reference. Also, photoconductor compositions comprising crystalline and co-crystalline mixtures of titanylphthalocyanine semiconductor pigment materials dispersed in an inert (non-conducting) polymeric binder have been described by Molaire and Keading U.S. Pat. Nos. 5,614,342 and 5,766,810, both incorporated herein by reference. While these aforementioned dispersions of semiconductor pigment in polymer binder are suitable for electrophotographic applications, these compositions are not sufficiently sensitive for photovoltaic and low-bias photodetector applications because the film-forming polymeric binder is electrically insulative.
For example, the inert polymeric binders used to prepare the electrophotographic photoconductor dispersions noted above include, for example, styrene-butadiene copolymers; vinyl toluene-styrene copolymers; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; vinylidene chloride-vinyl chloride copolymers; poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated polystyrene; poly(methylstyrene); isobutylene polymers; polyesters, such as poly[ethylene-coalkylenebis(alkyleneoxyaryl)phenylenedicarboxylate]; phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly[ethylene-coisopropylidene-2,2-bis(ethyleneoxyphenylene)-terephthalate]; copolymers of vinyl haloacrylates and vinyl acetate such as poly(vinyl-m-bromobenzoate-covinyl acetate); chlorinated poly(olefins), such as chlorinated poly(ethylene); cellulose derivatives such as cellulose acetate, cellulose acetate butyrate and ethyl cellulose; and polyimides, such as poly[1,1,3-trimethyl-3-(4′-phenyl)-5-indane pyromellitimide] and others. These polymers are not conjugated, do not contain an electrochemically active redox component, and as such are electronically insulating. Thus, as reported by Kitamujra and Yoshimura in: Proceedings of the Eighth International Congress on Advances in Non-Impact Printing Technologies, 1992, E. Hanson ed., IS&T, Springfield, Va., p237, for TiOPc dispersions in a polyester or polycarbonate, these pigment/polymer compositions exhibit low carrier mobility and require large applied electric fields in order to transport charge through the composition.
There is considerable published literature on photosensitive compositions that comprise n-type (electron-acceptor) organic semiconductor, such as C60 and its derivatives, dispersed in a p-type (electron donor) electronically conducting polymer, such as polythiophene or PPV and their derivatives. There is no prior art describing preparation or use of compositions that comprise n-type (electron-acceptor) organic semiconducto crystals dispersed in an n-type (electron donor) electronically conducting polymer. There are a few references; Hong, J.; Chen, H; Wang, M. J. Mater. Sci. 2003, 38, 4021, Nobutsugu, M.; Kanji, S. Kobunshi Ronbunshu 1983, 40, 211-216, Manabu, T.; Satoshi, O.; Hiroyki, T.; Seiko, N.; Hideo, N. Tech. Dig.-Int. Photovoltaic Sci. and Eng. Conf., 1st (1984) pp263-266, Takashi, K.; Akira, K.; Shunji, I. Denshi Shashin Gakkaishi 1984, 23, 18-23, that relate to the preparation and use of dispersions of a phthalocyanines pigment in the binder polymer of polyvinylcarbazole (PVK). Although not specified or demonstrated in these latter references, and although phthalocyanine pigments can exhibit n-type or p-type behavior depending on method of preparation and treatment, it may be considered that the phthalocyanine pigment is p-type in the above cited references and the PVK polymer is an electron donating polymer, hence comprising a p-type pigment dispersed in an electron donating (p-type) polymer. PVK polymer has a relatively high ionization potential, 5.8 eV as reported by Anderson et al, J. Am. Chem. Soc. (1998), 120, 9646-9655 and Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys. Lett. (1995), 67, 2281, whereas the typical ionization potential for a metal phthalocyanine is about 4.9 eV [4.8 eV for copper phthalocyanines as reported by Lee. S.; Wang, M.; Hou, X.; Tang, C. Appl. Phys. Lett (1999), 74, 670-672) and 5.03 eV for zinc phthalocyanine as reported by Kimura, T.; Sumimoto, M.; Sakaki, S.; Fujimoto, H.; Hashimoto, Y.; Matsuzaki, S. Chem. Phys. (2000), 253(1), 125-131]. Thus, in these cited examples, the ionization potential of the phthalocyanine differs substantially from that of PVK amount (the difference being greater than 0.5 eV). Hence, carrier (hole) mobility through the composition remains very poor because there is an energetic barrier (>0.5 eV) for the hole to move from the phthalocyanine to PVK.